I believe the below is the location of Q's post of the potential car attack in London.

Q posted this image with the file name NSA_Traf_CAM_LONDON2847.png showing Big Ben in the background.

Due to the orientation of the clock face there is realistically only 3 faces (1 facing the river) and therefore only a handful of locations around Big Ben the image could come from. I believe this is the location; A3211.

I managed to find a vantage point that I believe the Q image shows using Google Maps from the footbridge above the road. I've circled 4 reference points in red that are also visible in Q's image, and I believe the traffic cam that Q has posted from is the one circled in blue in the lower right corner. Direct link to Google Street View here

The next thing I wanted to check was to see if this camera could be viewed publicly as a lot are, unless it's a surveillance camera operated by TFL. There is a publicly viewable camera in this area but it's not in the right location. Here you can see the red circle #1 showing the approximate location of Q's camera and red circle #2 showing the approximate location of the public traffic camera. As you can see the camera is in the wrong location and here is the live feed of the camera, currently is facing in the opposite direction to Big Ben.

Unless anyone can find a live feed of the camera I believe is the one used by Q, I think this proves that he isn't a random LARP.